1. Field of the Invention
The present invention relates generally to detecting and locating ground faults in ungrounded electrical distribution systems and more particularly relates to detecting and locating ground faults without de-energizing the system.
2. Background Art
Most electrical distributions systems that are used to supply power to various types of equipment and machinery are designed as either “grounded” or “ungrounded” systems. A “ground fault” is an undesirable condition in an electrical distribution system, where the electrical current in the system flows or “leaks” outside of its intended flow path. Grounded electrical distribution systems are typically designed so that any electrical ground faults will trip one or more circuit breakers, thereby shutting down the electrical distribution system before any serious damage to people or equipment can occur. Electrical faults in most ungrounded electrical distribution systems will typically not shut the system down but will, instead, result in the generation of an alarm, thereby providing an alert while maintaining any critical loads in an energized condition. For this reason, an ungrounded electrical distribution system may offer increased reliability as a power source when compared to a grounded electrical distribution system.
Undesirable and unintended ground faults on ungrounded electrical distribution systems can result from many several different situations. For example, some major failures in electric equipment are caused by insulation breakdowns. Over time, the insulation on the electrical cables or wires can degrade, thereby exposing the conductors to other conducting materials, resulting in an unintentional grounding. Other types of failures that may introduce ground faults include circuit board failures, excess moisture being introduced into the electrical distribution system, bad wire joints and sleeves, leaking batteries, accidental grounds caused during testing, component failure, etc.
In most ungrounded electrical distribution systems, any ground fault that results in a detectable current flow due to faulted component in the system will activate a ground fault detection device (typically located at the main distribution bus), thereby alerting the operators of the electrical distribution system to the presence of the ground fault. Once detected, best practices dictate that the ground fault be located and eliminated so as to minimize the possibility of damage or harm to electrical equipment or human beings.
A typical ground fault detection circuit consists of a pair of resistors joined in series with the connection point between the two resistors being tapped, with the tap being connected to ground. This series combination of the resistors with a center tap is generally added in parallel to all of the other loads on the electrical distribution system. The use of such standard ground fault detectors in electrical distribution systems is well-known to those skilled in the art and, accordingly, is not discussed in greater detail herein.
Even more important and generally far more difficult than ground fault detection is the problem of ground fault location. While a ground fault detection system may accurately reveal that a ground fault exists on a given ungrounded electrical distribution system, the task of pinpointing the exact location of the ground fault is typically far more difficult to accomplish. Failure to quickly locate and rectify a ground fault may lead to equipment failure, inadvertent exposure to dangerous electrical environments, and other undesirable outcomes including reduction or termination of production or, in extreme cases, plant outages.
While it is important to locate ground faults as quickly and efficiently as possible, certain practical realities can complicate this task. For example, many circuits suffer from frequency induced “noise.” In these circuits, if a ground fault current at a normally detectable current level is present but yet lower in magnitude than the background noise on the electrical distribution system, traditional ground fault location equipment may not be able to distinguish between the actual ground fault and the ambient noise on the circuit. The result will be an inability to locate the circuit containing the ground fault. Accordingly, many traditional ground fault locating devices are be incapable of locating high resistance ground faults due to either a low amount of ground fault current, excessive noise on the system, or a combination of both.
In addition, with many large ungrounded AC electrical distribution systems, the lengths of the cable used in the distribution system can add capacitive reactance to the circuit. These long distribution runs, being capacitive in nature, can impede current flow in certain circuits, effectively creating the appearance of a ground fault where there is none. This can make the location of the actual ground fault far more difficult in some electrical distribution systems.
In addition to the capacitive resistance found in some systems, other electrical distribution systems sensitive equipment being fed by an ungrounded system, there may be capacitors used intentionally so as to create a path to ground. This situation is different than the capacitive resistance associated with long cable runs that coincidentally form capacitive paths to ground in that these are intentional pathways to ground. These capacitors are designed to be active only if there is any change in the current within a specific frequency range, such as background noise, on the power supply feeding certain sensitive equipment. If the background noise is in the targeted frequency range, it will be shunted to ground and back to the ground detector and to the source. This design provides the sensitive equipment with a much “cleaner” power supply and may be found on both AC and DC systems.
In other circumstances, intermittent, cycling or momentary ground faults or multiple ground faults on an ungrounded electrical distribution system may occur. An intermittent ground fault results from a ground fault occurring in electrical equipment during a specific operation but not in any specific time cycle. In an industrial setting, various types of equipment may periodically cycle between “on” and “off.” If this equipment also contains a ground fault, the detector will only sense the fault when the equipment is in the “on” position but not in the “off” position. During the time that the technician is investigating the ground fault, the strength of the ground fault may change or the ground may become intermittent, cycling or momentary stopping altogether, consequently making the entire location effort difficult if not futile. Similarly, other ground faults may be hidden in control circuit operations and may occur only during the transitory operation of a single switch.
Presently known portable ground fault locating techniques used on uninterruptible systems supplying vital loads typically attempt to locate the circuit containing the ground fault in one of two ways. First, by causing the ground fault current to vary in magnitude thereby providing a signal that can be detected by a Hall Effect sensor. The variation of the ground current may include the interruption of the ground current, in effect cycling the ground current from its full magnitude and then to zero, and then back again, thereby creating a “pulse.” These systems are only marginally effective and then only in conjunction with DC systems. Other devices may be deployed in similar fashion and may use a current transformer to detect the pulse.
In the second scenario for ground fault location, a separate signal (typically in the 30 Hz range) is “injected” as an artificial ground fault detection voltage signal. In these systems, the signal generator is coupled to the network at a first particular network location and generates for each line of the network an individual non-DC ground fault detection voltage signal between such line and ground. A current transformer or Hall Effect device is then used as a sensor to sense either the associated ground fault current changes or the artificially injected signal, systematically on every circuit of the system, until the circuit containing the ground fault has been located. This approach is very time consuming and leads to a great deal of “trial and error” searching, hoping to stumble across the appropriate branch circuit where the ground fault is located.
Thus, while certain techniques are available for detecting and locating ground faults in normally ungrounded electrical distribution systems, present systems and methods are sub-optimal due to the inherent limitations in both the equipment and techniques known to those skilled in the art. Accordingly, it would be an improvement in the art to augment or even replace current equipment and techniques for both ground fault detection and location.